This invention relates to uninterruptible power supplies which have a plurality of input power sources and which are operative to supply continuous power output irrespective of the condition of individual ones of the input power sources. More particularly, this invention relates to the power topology of such uninterruptible power supplies and, more particularly, to means for preventing momentary reductions in output voltage associated with step changes in output current, and means for monitoring the input voltages.
AC commercial power or an engine generator is often used as a primary power source to power communication and data processing equipment as well as other similar electronic equipment which utilize stored program controls or solid state integrated circuit technology. These circuits are generally very sensitive to variations in the input power signal from its desired standard wave form. Primary AC power wave forms are subject to many variations from the standard wave form due to the demands of other users on the power line, changes in demand of the power user, particularly large power users, and other factors. To avoid transient and momentary power outages or changes which may cause undetected damage to this equipment, or otherwise cause costly shutdowns due to damaged circuitry, disrupted communications or errors in computations, on-line uninterruptible power supplies are utilized to isolate variations in the AC power signal from the equipment being powered and to supply continuous power to an output load regardless of the actual performance of the basic input primary AC power signal or changes in the customer's load.
Some prior uninterruptible power supplies designed to serve this purpose have coupled two power sources through a single, high reactance ferroresonant transformer structure to supply uninterruptible power to the load to be energized. The primary power is typically that supplied by a commercial utility or generator. A secondary or reserve power supply supplies make up power to the load upon degradation of the primary power source. In the event of failure of the primary power source, the reserve power supply will supply all of the load. The high reactance transformer includes magnetic shunts for isolating the primary and the secondary sources. The transformer's high reactance also assists in providing the voltage differential necessary to regulate the output voltage of the transformer by supplying make up power from the reserve power supply when large voltage swings occur in the primary AC voltage.
The use of high reactance transformers however has inherent problems; a principal one of which is when there is a step change in the load connected to the load side of the uninterruptible power supply, the high reactance transformer causes a momentary reduction in the load voltage. This momentary reduction is defined by the voltage reduction which occurs when the load current goes from zero to full load. In the case of high reactance transformers, this momentary dip is on the order of 40%. This 40% dip in load voltage can cause significant problems. Circuitry designed to bypass the transformer has been used to help alievate this problem, but by-passing the transformer risks load voltage disruptions when the primary AC voltage is at an abnormal voltage. The use of high reactance transformers also reduces the uninterruptible power supply's efficiency and increases its size and the amount of heat losses.